What are these Play Script Collections?

Generally, we are talking about sets of short plays or sketches, usually with an overarching theme.
Some of these are designed to be performed together in a sketch show (a revue) - see, for example
'Acting Funny' or 'Skitskrieg'.
Others (particularly the collections of longer shows) might be performed separately,possibly forming a cycle of plays - for example,
Varadachary's Annotated Chess Masterpieces!
Anyway, regardless of these categories, one of the main points of a collection is thatYou get a discount!
Buying the collection will be cheaper than buying each of the plays separately (this makes a sketch show a much better economic proposition).